A dual-plasma [surface wave-coupled microwave (MW) at 2.45 GHz and capacitively coupled radio frequency (rf) at 13.56 MHz] reactor called MORFAX was developed for the deposition of amorphous wide band-gap alloys (a-SiOx:H, a-SiNy:H) at high growth rates and at low temperatures compatible with usual polymer substrates. These optical thin films may be used as components of multilayer coatings, such as index gradient devices or transparent interferential filters, to protect optical polymers. The dual-plasma MORFAX reactor was designed to provide three advantages : a high reactive species density (MW), a chemical selectivity due to the separation of the MW plasma and the rf plasma regions, and a variable ion bombardment energy due to the rf plasma de polarization. Using O-2, NH3, or N-2 gases diluted in He-Ar mixtures in the microwave plasma while silane was injected in the postdischarge region, we have obtained the following information. (i) Optical emission spectroscopy shows the changes in the electron energy distribution function for single- and dual-plasma modes as a function of the (He-Ar) mixture composition, that in turn provides a means to control the reactive gas processes. (ii) In the single MW mode, the decomposition of O-2 (diluted in He-Ar) is very efficient, producing an atomic oxygen density of [O] approximate to 10(14) cm(-3); due to low wall recombination, a high density of atomic O is transported in the flowing afterglow where silane is very strongly decomposed by O atoms. (iii) In the dual-plasma mode, atomic oxygen produced in the MW plasma and electrons due to the rf plasma both participate in the silane decomposition. Cooperative effects between the MW postdischarge and the rf plasma were observed, in particular on the deposition rate of silicon oxide films. (iv) As compared to oxide growth rates (up to 5 nm s(-1) limited by the silane depletion), the lower growth rate of silicon nitride films is attributed to the small reactive dissociation coefficient of SiH4 by N and NH2 radicals. (v) In situ and ex situ vibrational spectroscopy and spectroscopic ellipsometry show that the ion energy can be effectively used to increase the density of the oxide films. At 70 degrees C, good control of the stoichiometry and porosity of a-SiOx:H films was obtained over the full composition range (0 less than or equal to x less than or equal to 2.2). Water molecules are reversibly adsorbed upon exposure of the porous oxide films to the ambient and are desorbed under vacuum at room temperature.